The practice of chilling and freezing produce for both transport and storage has gained widespread acceptance and is the primary method of ensuring food preservation in the journey from the food processing plant to the customer. Bacterial growth poses a serious health risk for food which is not maintained at a safe temperature during the different stages in the supply chain, typically including the end of production line packaging, storage on the manufacturers' premises, transit, and arrival at the retailer.
As a visual inspection of frozen food is incapable of determining the core temperature, the current standard procedure to determine whether a product is correctly frozen requires insertion of a thermometer/thermal sensor through a hole drilled into the produce, and typically also through the product container. Such a procedure is time consuming, difficult to automate and detrimental to the product tested. Furthermore, as frozen food products are often transported between several locations and storage sites such as in the export of frozen meat from a remote country such as New Zealand, this invasive temperature testing procedure would need to be repeated on numerous occasions.
It is thus unsurprising that such invasive temperature testing procedures are sometimes omitted in practice with the resultant risks that foodstuffs may be either insufficiently frozen (with the corresponding health dangers) or alternatively that time and energy is wasted due to excessive freezing.
In order to address these difficulties, significant research has been devoted to non-invasive methods of temperature sensing. Microwave radiation interacts with organic matter in a manner which is particularly effective in relation to temperature measurement of frozen or chilled food.
Microwaves are a form of energetic electromagnetic radiation capable of penetrating matter to a degree dependant on the radiation energy and the matter composition. Although the strict numerical definition is subject to change by the Industrial, Scientific and Medical (ISM) standards, microwaves are currently recognised generally as radiation having a frequency of approximately 100 MHz-300 GHz. Microwave radiation is used as a heating medium due to its capacity for stimulation or vibration of water molecules in organic matter thus causing localised temperature increases.
However, the interaction of microwaves with organic matter is also temperature dependant. Microwave radiation is attenuated by passage through a given material according to a function which includes dependencies on both the temperature of the material and the frequency of the incident microwave radiation.
Significantly, the attenuation of the microwave radiation by the sample material drops sharply for temperatures below the freezing point, i.e., the material effectively becomes transparent to microwave radiation. This characteristic change in attenuation is a well-known feature of the interaction of microwaves and frozen produce.
Whilst this sharp change in transmittance (i.e. the inverse of attenuation) above and below the borderline of freezing temperatures may be used to indicate when a product is definitely frozen, determining the exact temperature of a lightly frozen or chilled item requires an accurate measurement system with an elimination of unknown variables and other error sources. It is known to use a sensor or detector to measure the un-attenuated microwave radiation passing through the sample as a fraction of the total microwave radiation transmitted by the microwave transmitter. This result may be used as a basis for calculating the temperature, or ‘ice-fraction’ of the sample.
However, the accuracy of such a system may be undermined by a number of factors. The microwave detector will be unable to discriminate between a microwave ray detected after transmission through the sample body and a ray which has been reflected from some other object, or even received directly from the transmitter without having been transmitted through or reflected from anything, including the sample. Without eliminating or otherwise accounting for such potential error sources such as alternate beam paths, the detector will receive a false reading, resulting in incorrectly calculated sample temperature.
Various solutions to this difficulty have been attempted.
Miyakawa, M. (1993) Tomographic measurement of temperature change in phantoms of the human body by chirp radar-type microwave computed tomography, [Med. & Biol. Eng. & Comput., 31, S31-S36.] developed a microwave-based computed tomography system to measure temperature in the human body. A pair of small (9.53 by 19.1 mm) antennae are rotated around a (phantom of a) body immersed in saline solution to measure the attenuation of the microwave signal at each step of the rotation. The individual measurements are then mathematically combined using a computer to generate an image of the microwave attenuation formed from two-dimensional slices through the body.
To eliminate the problem of alternate beam paths (i.e. paths not passing through the body) interfering with the measurement, Mikakawa (1993) uses a bath of saline. As saline has similar microwave attenuation to body fluid, immersing the body in saline minimises reflection and refraction as the beam enters the body. The body is then imaged at two different temperatures and the attenuations in the two images subtracted to show that they are different at different temperatures.
This technique has the drawbacks of;                the physical and practical inconvenience of using a saline bath to minimise reflection and refraction;        the requirement for a reference image to permit the subtraction of images to give a temperature measurement, and        a measurement time of at least several seconds.        
U.S. Pat. No. 5,341,814 Van De Velde et al (1994) teaches of a method for measuring the temperature of an object by detecting the thermal noise emitted by the object in the microwave frequency range. However, Van De Velde does acknowledge the following difficulty;                In this connection, there are known microwave radiometry devices in which the microwave radiation emitted via an antenna is picked up and the signals received are routed to signal processing means which enable the temperature of the object in question to be determined.        However, one of the main problems encountered in microwave frequency radiometry resides in the matching of the antenna in respect of the material the temperature of which one wishes to know. Indeed, the antenna used has a reflection coefficient Ro and, as a result, the antenna is never perfectly matched, given that the objects to be measured generally have different configurations, sizes and properties.        
U.S. Pat. No. 4,346,716 Carr (1980) relates to the detection of tumours by measuring the differential rate at which tumours are heated by microwave energy compared with normal tissue, due to the fact that tumours are not cooled as effectively as normal tissue by flowing blood. The temperature measurement mechanism is based on passive measurements of microwave emissions at 4.7 GHz, (as per Van De Velde et al), rather than the transmittance measurement.
U.S. Pat. No. 4,870,234 Steers et al (1989) relies on the same fundamental mechanism discussed earlier to distinguish frozen from unfrozen product, i.e. a measurement of the different microwave transmittance through material dependent on whether it is frozen or unfrozen.
The Steers et al approach uses the rate of change in temperature of a reference material to measure the amount of microwave energy passing through the product rather than a microwave receiver. This method has the advantage of low cost and may provide an appropriate alternative in some applications. However, for applications requiring temperature measurement of products in systems with a high throughput rate (such as in a meat processing plant), the method of Steers et al suffers from three serious disadvantages:    1. a single temperature measurement requires sufficient time for the reference material to heat measurably—requiring seconds or even tens of seconds and thus limiting throughput;    2. the use of a microwave transmitter with sufficient power to measurably heat the reference material necessitates appropriate shielding structures which are generally inconvenient or impractical to implement in automated food processing plants, and there is no provision disclosed for cooling the reference sample to facilitate rapid repeat temperature measurements.    3. Irrespective of a possible resolution of the first two disadvantages listed above, the third disadvantage would still prevent practical temperature measurements in a multi-sample temperature measurement scenario. Due to the absence of any cooling, the reference sample would eventually be heated to an equilibrium temperature from repeated measurements, thus preventing further reference sample temperature change implicit for any sample temperature measurements.
The ‘Celsius’ unit by Loma Systems™ is a microwave temperature measurement device, resembling a domestic microwave oven. After a sample is manually placed inside the fully enclosed cabinet (via a front door), the sample is irradiated by microwave radiation and a temperature measurement is taken, though the specific measurement mechanism used is not disclosed by Loma Systems™ in their promotional literature.
However, from the configuration of the device, it seems likely a similar system to that of Van De Velde et al is utilised, i.e. the temperature is calculated from the microwave radiation emitted or reflected from the irradiated object.
If such a mechanism is employed, the detector also receives microwaves which have been deflected, reflected and refracted from the enclosure interior. The temperature measurement thus needs to take account of all the radiation detected and not just those microwaves emanating from the sample.
Whilst this system may be suitable for measuring samples small enough to fit in the enclosure, this system does not lend itself to multi-sample measurements in high throughput applications.
A housing completely enclosing each sample is an unavoidable requirement as the microwave measurements would otherwise be affected by stray environmental electromagnetic radiation. Such a requirement would necessitate complex and costly mechanical systems to repeatedly manipulate samples from a production line into a measurement housing, seal the enclosure, rapidly perform the temperature measurement, extract the sample and return to the production line.
It can be seen therefore that none of the above prior art provides a practical, non-invasive means suitable for incorporation in rapid sample throughput systems for determining the temperature of chilled or frozen produce or other water-rich substances.
Alternative forms of penetrative electromagnetic radiation may also be transmitted through a sample for a variety of reasons, e.g. to analyse the sample's constituents, or to measure the degree of transmission/attenuation of the sample to particular frequencies, to provide a heating effect, or the like. In such applications, it may be important to reduce measurement uncertainties caused by detecting radiation reflected via some circuitous route (e.g. from environmental conditions or structures) rather than was transmitted directly through the sample. Such non-microwave electromagnetic radiation (ranging from radio frequency waves to higher frequency radiation) may also suffer from the aforesaid disadvantages of microwave temperature measurement systems.
All references, including any patents or patent applications cited in this specification are hereby incorporated by reference. No admission is made that any reference constitutes prior art. The discussion of the references states what their authors assert, and the applicants reserve the right to challenge the accuracy and pertinency of the cited documents.
It will be clearly understood that, although a number of prior art publications are referred to herein, this reference does not constitute an admission that any of these documents form part of the common general knowledge in the art, in New Zealand or in any other country.
It is acknowledged that the term ‘comprise’ may, under varying jurisdictions, be attributed with either an exclusive or an inclusive meaning. For the purpose of this specification, and unless otherwise noted, the term ‘comprise’ shall have an inclusive meaning—i.e. that it will be taken to mean an inclusion of not only the listed components it directly references, but also other non-specified components or elements. This rationale will also be used when the term ‘comprised’ or ‘comprising’ is used in relation to one or more steps in a method or process.
It is an object of the present invention to address the foregoing problems or at least to provide the public with a useful choice.
Further aspects and advantages of the present invention will become apparent from the ensuing description which is given by way of example only.